Mapping water in protostellar outflows with Herschel PACS and HIFI observations of L1448-C
Artikel i vetenskaplig tidskrift, 2013

Context. Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play. Aims. Our goal is to investigate the spatial and velocity distribution of H2O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H2O emitters among the class 0 outflows. Methods. To this end, maps of the o-H2O 1(10)-1(01) and 2(12)-1(01) transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H-2 S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H2O column densities were inferred using large velocity gradient radiative transfer calculations. Results. The water distribution appears to be clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557 GHz line is confined to radial velocities in the range +/- 10-50 km s(-1), but extended emission at extreme velocities (up to v(r) similar to 80 km s(-1)) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H2O 1(10)-1(01)/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H2O lines and the comparison with H-2 indicate averaged T-kin and n(H-2) values of similar to 300-500 K and 5 x 10(6) cm(-3), respectively, while a water abundance with respect to H-2 of about 0.5-1x10(-6) along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H2O chemistry. Conclusions. The results of our analysis show that the bulk of the observed H2O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H2O gas in the post-shock region. Among the possible scenarios, we favor H2O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength.

high-velocity sio

molecular bullets

ISM: molecules

submillimeter

ISM: abundances

h2o

ISM: jets and outflows

stars: formation

outflows

stars: winds

interstellar clouds

chess spectral survey

h-2 emission

star-forming regions

gas

shock-waves

Författare

B. Nisini

Osservatorio Astronomico di Roma

G. Santangelo

Osservatorio Astronomico di Roma

S. Antoniucci

Osservatorio Astronomico di Roma

M. Benedettini

Istituto Nazionale Di Astrofisica, Rome

C. Codella

Osservatorio Astrofisico Di Arcetri

T. Giannini

Osservatorio Astronomico di Roma

A. Lorenzani

Osservatorio Astrofisico Di Arcetri

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

M. Tafalla

Observatorio Astronomico Nacional

Per Bjerkeli

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

S. Cabrit

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

P. Caselli

Osservatorio Astrofisico Di Arcetri

University of Leeds

L. Kristensen

Leiden Observatory Research Institute

D. A. Neufeld

Johns Hopkins University

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

E. F. van Dishoeck

Max Planck Institut fur extraterrestrische Physik

Leiden Observatory Research Institute

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 549 A16

Ämneskategorier

Astronomi, astrofysik och kosmologi

DOI

10.1051/0004-6361/201220163